1. Field of the Invention
This invention relates to a wafer used, for example, in semiconductors and a method of producing the same, and more particularly to a technique of producing the wafer in which a projection(s) liable to be caused at a rear face of the wafer due to the holding with a wafer-holding jig (hereinafter referred to as a susceptor) in a heat treating step of the wafer can be removed effectively while maintaining a good flatness.
2. Description of Related Art
Recently, the wafer, for example, so-called one-side mirror-finished wafer is particularly demanded to have a severer surface precision from semiconductor device makers. In the production of such a wafer, it frequently tends to conduct a heat treatment (annealing) at a higher temperature above 1100° C. such as treatment for epitaxial growth, annealing treatment accompanied with the miniaturization of the device. In case of conducting such a heat treatment, a micro-projection(s) is easily formed on the rear face of the wafer by taking away a part of a susceptor directly contacting with the rear face of the wafer due to the holding of the wafer in a heat treating furnace. Also, as the heat treating temperature rises, the projection becomes large to finally form a micro-projection having a projection height of not less than 1 μm.
If the micro-projection (having, for example, a height of about 0.5-10 μm) is existent in the rear face of the wafer, when the wafer is chucked under vacuum at a photolithography step in the production of the device, the surface of the wafer is rendered into a somewhat convex-curved face because the completely flat face can not be held. Therefore, the defocusing in the exposure is caused at the surface of the wafer irradiated by the light and the exposure can not be conducted in a high precision, so that there is a fear of lowering the product yield.
For this end, it is required to remove the micro-projections from the wafer by subjecting the rear face to a polishing.
As means for polishing the rear face of the wafer, there are mentioned, for example, a so-called loose abrasive polishing method wherein the rear face of the wafer is polished while feeding a slurry containing loose abrasives such as colloidal silica or the like to a working face of an abrasive cloth of an expandable urethane type obtained by slicing a block of a foamed urethane or an abrasive cloth of a porous non-woven cloth type obtained by impregnating polyurethane into polyester felt as disclosed in JP-A-2005-5490, and a polishing method using a bonded abrasive as disclosed in JP-A-2003-257905, JP-A-2004-337992 and JP-A-2005-129644.
In the polishing method using the loose abrasives as disclosed in JP-A-2005-5490, the micro-projections existing on the rear face of the wafer can be removed as far as they have a certain height (e.g. a height of less than 1 μm). However, when the micro-projections have a height exceeding 1 μm, it is difficult to completely remove such micro-projections.
Also, when the micro-projection having, for example, a height of 1 μm is completely removed by the loose abrasive polishing method, it is required to polish the rear face of the wafer at a total polishing amount of not less than 3 μm. As the total polishing amount becomes large, the flatness is undesirably deteriorated. This tendency becomes particularly remarkable when the size of the wafer is increased from 200 mm to 300 mm together with the rise of the heating temperature for the wafer. Therefore, it is required to develop means for maintaining the flatness by removing the micro-projection having, for example, a height of not less than 1 μm at a less total polishing amount (e.g. not more than 1 μm).
On the other hand, the method using the bonded abrasive as disclosed in JP-A-2003-257905, JP-A-2004-337992 and JP-A-2005-129644 is developed for the purpose of maintaining or improving the polishing performance, but does not utterly consider a point that the micro-projection is removed from the rear face of the wafer at a total polishing amount as small as possible for maintaining the flatness.